Ugh! The whole carry on issue just kills me. It's a huge reason why I now book exclusively Delta Comfort+ (dedicated carry on bins) or JetBlue Even More Space (first to board the plane after veterans and Mosaics) seats. I had a horrible experience with American Airlines trying to tell me I needed to gate check my small, soft duffel after 1) I paid for early boarding for the express reason of wanting early access to overhead bins and they decided to just start letting people on the plane and not board by proper zone 2) They let multiple people with 3-4 bags a piece ahead of me. I absolutely hate that I have to play by airlines' kind of insane rules (high checked bag fees, etc.) in order to store my carry-on in the overhead bin, but it's worth it for the "guarantee" and peace of mind.
After that aforementioned incident, in which AA gate agents were incredibly rude and used profanities at me, I officially boycotted them for life and decreed my Delta C+/JetBlue EMS exclusivity clause. Regardless of price or flight times. I make it work with one of these airlines when I travel for pleasure or business. Not worth the angst of the unknown.
Sorry for the tangent, OP!! I'd love to hear if you were able to keep that middle seat!